1. Field of the Invention
This invention relates to a reinforced plastic valve and, more specifically, to a polyethylene plastic valve which has anti-creep properties and is capable of operating at higher pressure ratings for an extended period of time.
2. Description of the Prior Art
Plastic valves, such as those disclosed in U.S. Pat. Nos. 4,014,513; 4,171,711; 4,234,011; and 4,488,741, have recently been satisfactorily and successfully employed for the flow control of numerous types of fluids in various piping systems and in a wide range of environmental conditions. However, because of the nature of plastic, there have heretofore been some limitations on the amount of fluid pressure which should be allowed in systems which employ plastic valves. For example, it has been found that, when various plastic valves have been utilized in systems which have a relatively high operating pressure, after an extended period of time, the valve plug and/or valve body can experience "creep" which alters the design dimensions of the valve and/or plug to decrease its overall efficiency and reliability.
"Creep" can be defined as progressive strain without increased stress. If one is free to select alternative materials of construction, it is possible to eliminate any real concern for "creep". However, there are instances where the plastic body material must be identical to that of the piping system. For example, if the body is to be fused to the pipes in the system, the same material is required for a proper union. The piping could display high "creep" characteristics and still be reliable while the same "creep" in the body could alter its dimensions and reduce the reliability of sealing around the plug which prevents leakage and sealing at the valve seat which controls flow through the valve.
Although not specifically related to the type of valve of the present invention, U.S. Pat. No. 3,092,365 discloses a rubber plug for a valve which is basically formed around and reinforced by an internal metal structure. Other types of valves have employed inserts in various types of closure members and in various areas of the valve body to generally solve different problems than that of "creep" as described hereinabove. For example, U.S. Pat. No. 3,133,722 discloses a sleeve liner which can be made of plastic material but is intended to serve as a backing material for a tapered plug member which is primarily intended to function as the closure device for the valve. U.S. Pat. No. 4,303,223 discloses valve facing strips of a fire hydrant valve which are primarily intended to provide a reliable, low friction working surface for the axial movement of the valve disclosed therein.
Additional patents disclose some form of sleeve or reinforcing element which are not primarily intended to strengthen the valve body but are instead intended to insure proper sealing of the valve during closure. For example, U.S. Pat. No. 3,061,269 discloses an internal sleeve which can be made of numerous types of material but is primarily intended to provide a surface having a low-coefficient of friction for the rotation of the valve closure member disclosed therein. Similarly, U.S. Pat. Nos. 3,244,389 and 3,398,925 disclose ball valves which have annular reinforcing means at the edge seals thereof to insure that proper sealing is maintained against the surface of the ball to prevent the loss of fluid from the flow passages thereof.
U.S. Pat. No. 3,091,428 discloses a metallic ball valve and U.S. Pat. Nos. 3,192,945 and 3,913,610 disclose metallic butterfly valves which are primarily intended for use in metal piping systems. The ball valve of U.S. Pat. No. 3,091,428 includes a central cylindrical ring section which primarily supports the ball and to which identical cups are joined to form the composite valve. Similarly, the butterfly valves of U.S. Pat. Nos. 3,192,945 and 3,913,610 disclose cylindrical metallic sleeves or body portions which primarily rotatably support the butterfly closure members thereof. These valves include the provision of a lining or interior coating for the cylindrical metallic portion which is formed of resilient rubber-like, plastic, or synthetic material to provide the sleeve with erosion and/or corrosion protection.
More significantly, U.S. Pat. No. 4,348,006 discloses a butterfly-type valve having a plastic body portion with a valve seat formed by a metal sleeve molded within and sealed to the plastic valve body portion with an annular rubber sleeve insert vulcanized to the metal sleeve. The metal sleeve is intended to prevent migration or "creep" of the elastomeric insert but an additional sealing means between the metal sleeve and the plastic body portions is employed to insure that there will be no fluid leakage therebetween.
Finally, although not intended to reinforce the valve body thereof or to qualify the valve for use with fluids operating at higher pressures, U.S. Pat. No. 3,526,386 discloses a plastic valve having a metallic sleeve which is inserted in and embedded within the flow line thereof. The sleeve is said to "reinforce" the flow line but is primarily utilized to provide a stronger, reinforced coupling means for installing the plastic valve within a piping system.
U.S. Pat. No. 4,171,711 also discloses a bottom cap member to permit leakage if the fluid pressure within the valve body increases above a given amount. U.S. Pat. No. 4,171,711 also discloses valve seats positioned with the cylindrical plug with key portions as guides for the seal.
U.S. Pat. No. 4,511,120 discloses a rotary plug type plastic service valve that has a unitary plug and actuator member where the actuator portion of the valve has a flange portion so that impact forces applied to the actuator are transmitted away from the stem and plug portions and distributed throughout the valve body.
Consequently, there remains a need for providing a plastic valve which includes means for insuring that the valve can be satisfactorily employed in fluid systems of higher pressure without any detriment thereto over an extended period of time.